4KQ starter, Oxy sensor

[DQVT74A@prodigy.com ( CHANGLONG HUNG)]

Hi all, 

OK, after an eventful Saturday, I woke up to a car that refuse to 
start on Sunday.  The symptom is car cranks but won't turn over.  It 
has been happening on and off.  Before, all I had to do is give a 
little gas and the car starts fine.  Once it starts, I have no 
problem restart as long as the engine is warm.  Is the starter going 
bad, or should I replace the battery first?  Also, the car hesitate 
at low end from 1500-3000rpm.  Once it gets above 3000rpm, it pulls 
well.  I was thinking perhaps oxy sensor needs to be replaced.  How 
much should I expect to pay for starter and oxy sensor?   Any expert 
opinions?  TIA

Sammy
'874KQ-not too healthy